The present invention relates to a method for manufacturing outlet nozzles for rocket motors, according to the preamble to the subsequent claim 1. The invention is especially intended for use in respect of cooled outlet nozzles for rocket motors driven by liquid fuel.
Rockets is the collective name given to those crafts which comprise a drive source in the form of a rocket motor. Rockets are currently used, inter alia, for space-flights and in this context, for example, for research and for communications purposes. Rocket motors are also used in other contexts, for example in apparatus for aiding the take-off of aircraft and for ejecting the pilot from an aircraft in emergency situations.
In a rocket motor, the energy for propulsion of the rocket is generated in a combustion chamber through the burning of a fuel, for example in the form of liquid hydrogen. This fuel is fed together with an oxidizer (for example in the form of liquid oxygen) via valves to the combustion chamber. As the fuel is burnt, combustion gases are generated in the combustion chamber. These combustion gases flow out rearwards from the combustion chamber and out through an outlet nozzle, whereupon a reaction force is generated such that the rocket is propelled forwards. The outlet nozzle is configured to allow expansion and acceleration of the combustion gases to a high velocity such that the necessary thrust is attained for the rocket. The fact that a rocket motor can generate very large drive forces and can additionally operate independently of the surrounding medium makes it especially suitable as a means of transport for space-flights.
Whilst a rocket motor is running, the outlet nozzle is subjected to very high stresses, for example in the form of a very high temperature on its inside (in the order of magnitude of 800 K) and a very low temperature on its outside (in the order of magnitude of 50 K). As a result of this high thermal load, stringent requirements are placed upon the choice of material, design and manufacture of the outlet nozzle. Not least, there is a need for effective cooling of the outlet nozzle.
In order to achieve optimal cooling, the outlet nozzle according to the prior art is configured with a number of cooling ducts which are arranged in parallel within the actual nozzle wall and extending between the inlet end and the outlet end of the outlet nozzle. The manufacture of the outlet nozzle, that is to say the configuration of its wall such that the necessary cooling ducts are formed, can be carried out using a host of different methods.
In this context, it is also the case that high efficiency can be obtained in a rocket if the cooling medium is also used as fuel. For this reason, there is often a desire to re-use all the cooling medium for burning in the combustion chamber.
A previously known method of manufacturing a cooled outlet nozzle is by configuring the nozzle wall from a large number of round or oval pipes made of, for example, nickel-based steel or stainless steel, which pipes are arranged close together and are subsequently joined together along their sides. This joining can in this case be realized by means of soldering, which is however a manufacturing method which is relatively costly. Moreover, the soldering results in an increase in weight of the outlet nozzle. The soldering additionally represents a complicated and time-consuming operation in which it is difficult to attain the necessary strength and reliability in the completed wall structure.
Another significant drawback with solder-based joining is that it is complicated and expensive to check the solder joints. If, for example, a fault occurs along a solder joint, it is very difficult to repair the joint since this damage is not normally accessible. Furthermore, the soldering structure is relatively weak in the tangential direction, which in certain cases can create the need for a strengthening structure in the form of a jacket. This is especially the case in those instances in which the flame pressure during the combustion in the rocket motor is very high or in which high lateral forces are present.
A manufacture which uses soldering can further place a limit upon the maximum temperature at which the outlet nozzle can be used.
An alternative method of manufacturing a cooled outlet nozzle is by diffusion-welding of round or rectangular pipes which are arranged in parallel. Even though this method has advantages over the soldering method, it is still relatively expensive.
According to a further manufacturing method, rectangular pipes of constant cross section made from nickel-based steel or stainless steel are used, which pipes are arranged parallel with one another and are welded together. The pipes are spirally wound such that they form an angle with the geometrical axis of the nozzle, which angle increases progressively from the inlet end of the nozzle to its outlet end to form a bell-shaped nozzle wall. The abovementioned joining method has the drawback that those types of rectangular pipes which are commercially available for use with this method are normally made with constant wall thickness. This means that the wall structure of the outlet nozzle cannot be configured for an optimal cooling capacity, since the walls between two mutually adjacent cooling ducts are unnecessarily thick. Moreover, the spiral winding means that the cooling ducts are long and hence give rise to an increased fall in pressure, which for certain running states of the rocket motor is undesirable.
A further method for manufacturing a combustion chamber for rocket motors s described in patent document U.S. Pat. No. 5,233,755. According to this method, a corrugated structure is used to form an inner wall, which is joined together with an outer wall by, for example, soldering, diffusion-welding or laser-welding. Cooling ducts are thereby formed, through which a cooling medium can be conducted.
A drawback with the method according to U.S. Pat. No. 5,233,755 is that, owing to the configuration of the corrugated inner wall, xe2x80x9cpocketsxe2x80x9d are formed at its points of contact against the outer wall. In these portions, a limited flow of the cooling medium is therefore obtained, resulting in locally reduced cooling of the wall structure. This gives rise, in turn, to a risk of overheating of the wall structure. There is additionally a risk of dirt, for example in the form of small particles, accumulating in these pockets. This dirt may subsequently be released from the cooling ducts, which is also a drawback, especially if the cooling medium is also to be used as fuel in the rocket motor.
A further drawback with the manufacturing method according to U.S. Pat. No. 5,233,755 is that the corrugations in the inner wall lead to a limited part of the cooling medium being allowed to have contact with the inner, warm nozzle wall. This too adversely affects the cooling. Furthermore, the corrugated structure is subjected to bending forces owing to the pressure of the cooling medium inside the structure. Together with the sharp notch at the respective welding joint, these bending forces lead to very high stresses upon the wall structure. This type of structure therefore has limits in terms of its pressure capacity and working life.
The corrugated shape of the distancing material, compared with straight, radially directed distancing elements, leads moreover to increased weight and increased flow resistance.
The object of the present invention is to make available an improved method for manufacturing a cooled outlet nozzle for a rocket motor. This is achieved by means of a method, the characterizing features of which can be derived from subsequent claim 1.
The invention relates more precisely to a method for manufacturing an outlet nozzle in rocket motors, which outlet nozzle is configured with a wall structure which comprises a plurality of mutually adjacent cooling ducts extending substantially from the inlet end of the outlet nozzle to its outlet end, which method comprises positioning of an outer wall around an inner wall, configuration and positioning of a plurality of distancing elements between the said outer wall and the said inner wall, and joining of the said distancing elements between the said inner wall and the said outer wall, whereupon the said cooling ducts are formed, The invention is characterized in that the joining is realized by means of laser-welding and is designed for the configuration of weld joints which, in a cross section through the wall structure, are substantially T-shaped and have a shape which is rounded towards the inside of the cooling ducts.
As a result of the invention, a host of advantages are obtained. Firstly, a manufacturing method for an outlet nozzle is provided, which can be executed at low cost. Moreover, as a result of the specific geometry in the weld joints, a wall structure is obtained having good nozzle-cooling properties. Moreover, the rounded shape of the weld joints produces an even flow in which there is very little risk of an accumulation of particles.
A further advantage with the invention is that the distancing elements, as a result of their flat configuration in the radial direction in relation to an imaginary axis of symmetry through the outlet nozzle, are not subjected to any bending forces. In addition, a very low concentration of stresses in the weld joints is obtained, owing to the configuration of the radii between the integral components.
The invention further offers low material consumption, low weight, low costs, high reliability and good thermal cooling capacity of the complete wall structure. Moreover, the geometry of the wall structure can be easily adapted to the cooling requirements which pertain to the particular application.
Advantageous embodiments of the invention can be derived from the subsequent contingent claims.